Add override using environment variable for freestyle.sh endpoint (#853)

Hi there, I have noticed that the project recently moved to using
freestyle.sh for rendering emails. I am currently self-hosting stack
auth and freestyle.sh does not have a privacy policy as far as I can
see. Therefore, it is not usable in a compliant way.

This PR simply adds an optional environment variable to be able to
override the API endpoint. This way, users can easily opt-out and use a
different implementation to render the mails.

Let me know if this is something you would consider merging and if the
variable should be added in some docs or examples.
<!-- ELLIPSIS_HIDDEN -->

----

> [!IMPORTANT]
> Add environment variable `STACK_FREESTYLE_API_ENDPOINT` to override
Freestyle API endpoint in `freestyle.tsx`.
> 
>   - **Behavior**:
> - Adds environment variable `STACK_FREESTYLE_API_ENDPOINT` to override
Freestyle API endpoint in `Freestyle` class constructor in
`freestyle.tsx`.
> - Defaults to `http://localhost:8122` for development and test
environments if API key is `mock_stack_freestyle_key` and no override is
set.
>   - **Imports**:
> - Adds `getEnvVariable` import from
`@stackframe/stack-shared/dist/utils/env` in `freestyle.tsx`.
> 
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<!-- ELLIPSIS_HIDDEN -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Freestyle service endpoint can be configured via an environment
variable for deployments.
* By default the configured endpoint is used; in development/test, a
mock key causes the client to point to a local default for local
testing.
* **Bug Fixes**
* Using the mock key outside development/test now triggers a validation
error to prevent accidental misuse.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Paul Graf <paul.graf@student.tugraz.at>

apps/backend/src/lib/freestyle.tsx

@@ -10,7 +10,7 @@ export class Freestyle {
 
   constructor(options: { apiKey?: string } = {}) {
     const apiKey = options.apiKey || getEnvVariable("STACK_FREESTYLE_API_KEY");
-    let baseUrl = undefined;
+    let baseUrl = getEnvVariable("STACK_FREESTYLE_API_ENDPOINT", "") || undefined;
     if (apiKey === "mock_stack_freestyle_key") {
       if (!["development", "test"].includes(getNodeEnvironment())) {
         throw new StackAssertionError("Mock Freestyle key used in production; please set the STACK_FREESTYLE_API_KEY environment variable.");